1. Field of the Invention
The present invention relates to a writing device for writing, on a recording medium, record data using a beam of laser light modulated in accordance with the record data (i.e., based on optical modulation recording).
2. Description of the Related Art
When data is written on a recording medium, such as an optical disk, based on optical modulation recording, laser light is usually emitted in the form of pulses to carry out thermal control for satisfactory shaping of pits (marks) formed on the disk.
More specifically, a laser irradiation period is controlled by setting a pulse waveform to be suitable as a laser driver pulse for driving a laser, and controlling each pulse duration in the direction of the time base.
As data writable disk media, there are known write-once disks such as CD-R (CD-Recordable=CD-WO) and rewritable disks such as CD-RW (CD-Rewritable). In these CD disks such as CD-R and CD-RW, it has been customary that an EFM signal is created as record data and a laser drive pulse is formed in accordance with the EFM signal.
A pulse width of the EFM signal is specified so as to fall in the range of 3T-11T. “T” corresponds to one clock period at the EFM frequency.
When writing data in CD-R based on changes of a dye film, for example, laser drive pulses shown in FIG. 14(b) are created depending on lengths of pits and lands to be recorded, as shown in FIG. 14(a), and a laser is driven by the laser drive pulses to emit light. Additionally, a level PWr in the drawing corresponds to a laser recording power.
In some CD-Rs, step-shaped laser drive pulses shown in FIG. 14(d) are created by combining two kinds of pulses shown in FIGS. 14(b) and 14(c) with each other. With this method, the laser power is increased up to a level PWod, for example, in part of duration of a pulse for producing a pit. Such a part of the pulse duration is also called an overdrive pulse. Addition of an overdrive pulse enables a laser light level to be more finely controlled within the pulse duration.
When writing data in CD-RW by the phase change recording method, a laser is driven by generating laser drive pulses, called a pulse train, having a laser power level repeatedly changed between recording power Wr and cooling power PWc within a pit forming zone, as shown in FIG. 14(e). In a land period, the laser power has a level of erasure power PWe.
The above-mentioned control of laser drive pulses for CD-R and CD-RE in the direction of the time base is effected by controlling each pulse waveform at its rising and falling edges, for example, indicated by o in FIGS. 14(b) to 14(e).
In other words, as shown in FIG. 15, the rising and falling edges of each pulse waveform are controlled with delay processing DL such that the phase advances or retreats.
The reason why a pulse waveform is controlled in the direction of the time base is as follows.
In a write-once disk such as CD-R, for example, as a pit to be recorded becomes longer, it is required to increase the laser recording power relative to the reading power for a longer period of time. Therefore, a larger amount of heat is accumulated in a recording layer, and an area subjected to chemical changes is enlarged, whereby a pit actually recorded tends to have a greater length than the prescribed one. This tendency is inevitably more significant as a recording layer of the disk has higher heat sensitivity or higher heat conductivity.
Further, the length over which a pit going to be now recorded is actually formed also depends on the length of a land just before the pit. Stated otherwise, the smaller the length of a land just before the pit, the less is radiated heat accumulated during recording of a preceding pit and the more significantly is affected the relevant pit by thermal interference.
For example, even when a pit going to be recorded has the same length and a beam of laser light is irradiated with the same power to record the pit for the same period of time, the pit actually formed tends to have a greater length as a land just before the relevant pit has a smaller length.
In view of the above-described situations, to cope with the former problem, a laser drive pulse is controlled in the direction of the time base such that the laser drive pulse has an optimum length depending on the length of a pit going to be recorded (i.e., the pit length in the range of 3T-11T). To cope with the latter problem, a laser drive pulse is controlled in the direction of the time base such that the laser drive pulse has an optimum length depending on the length of a land just before the pit going to be recorded. To cope with a combination of the former and latter problems, a laser drive pulse is controlled in the direction of the time base depending on combination of both the length of a pit going to be recorded and the length of a land just before the pit.
The control of a laser drive pulse in the direction of the time base is carried out by delay processing utilizing a PLL clock in sync with a signal to be recorded, or delay processing using a delay line.
FIG. 16 shows one example of a delay circuit using a delay line. The delay circuit comprises, for example, five stages of delay gates 101-105 and a selector 100.
In this delay circuit, a required delay time can be obtained by the selector 100 selecting one tap in accordance with a control signal (not shown). Assuming, for example, that one delay gate has a delay time of 5 nsec, a total delay time can be changed over in units of 5 nsec within the range of 0-25 nsec. Thus, an appropriate length of the laser drive pulse can be realized by changing a tap selected by the selector 100 depending on the length of a pit going to be recorded or the length of a land just before the pit.
FIG. 17 shows one example of a delay circuit for delaying a pulse by a shift register 110 using a PLL clock CL. Assuming, for example, that the frequency of a PLL clock is 200 MHz, a time period of one clock is about 5 nsec and therefore delay processing can be performed in units of 5 nsec. If the frequency of a PLL clock is 400 MHz, delay processing can be performed in units of 2.5 nsec.
With a recent increase in writing rate, data has become written on CD-R and CD-RW at 2- and 4-times speeds rather than a conventional rate (1-time speed). A further increase in writing rate up to 8-time, 12-times or more speeds is now under the progress.
Considering the above-mentioned control of a laser drive pulse in the direction of the time base in relation to a writing rate, control of a delay time in units of 5 nsec, for example, is satisfactory for writing at a 1-time speed. However, when such control is applied to writing at a 4-, 8- or 12-time speed, the accuracy of a delay time becomes insufficient and the laser drive pulse cannot be controlled in the direction of the time base with high accuracy.
Writing at an 8-time speed, for example, requires a delay time to be controlled in units of at least 2.5 nsec. Also, in consideration of writing at a 12-time speed, control of a delay time in units of 1 or less nsec is demanded.
In trying to perform control of a delay time in units of 0.5 nsec by using the delay line shown in FIG. 16, for example, the control can be realized by employing delay gates each of which has a delay time of 0.5 nsec.
Also, in the case of employing the delay circuit utilizing a PLL clock shown in FIG. 17, for example, the control can be realized by increasing the frequency of the PLL clock.
However, increasing the PLL clock frequency up to a sufficient level is difficult to implement in practice. It is therefore more realistic to employ a delay line.
There is however another situation that even writing devices adapted for 8- and 12-time speeds are practically required to support once-speed writing as well.
Assuming that control of a delay time variable in the range of 0-25 nsec is required for once-speed writing, 50 stages of delay lines are needed in the case of employing a delay line in which a delay time is variable in units of 0.5 nsec. Of course, a larger number of gate stages are necessary when a unit of a variable delay time, i.e., a delay time of one gate stage, is set to a smaller value, or when variations in delay time of each gate stage is taken into consideration.
In other words, control of a laser drive pulse in the direction of the time base can be relatively easily realized with high accuracy in a manner adapted for high-rate writing by employing a delay line. However, trying to support once-speed writing as well by the same delay line gives rise to a problem of a difficulty in realizing such a delay line because a very large number of gate stages are required.
Another problem is that, since a delay line is generally susceptible to large variations in device accuracy, e.g., in delay time depending on temperatures, it is hard in the delay line to realize control of a laser drive pulse in the direction of the time base with high accuracy.